In satellite based communication systems employing multiple relay satellites, it is often required to communicatively link at least two of the satellites in order to facilitate global communications between stations located, for example, on opposite sides of the earth. These relay satellites typically reside in synchronous orbit at an altitude of approximately 22,300 miles. The long distances between these synchronous orbit satellites can impose costs relating to the high powered transmissions and the large antennas required to support the links between these satellites.
The use of known types of lower altitude relay satellite systems can also involve drawbacks. By example, lower altitude relay satellites can suffer from discontinuous inter-connections and also may provide only limited coverage.
Therefore, it is desirable to provide a satellite relay communication system which is located at a lower altitude than synchronous orbit, and which thus shortens the communication range while providing nearly global coverage.
U.S. Pat. No. 5,439,190, issued to Horstein et al., discloses a satellite-based cellular telecommunications system employing a constellation of telecommunications satellites in medium earth orbit. The system provides multibeam radio frequency communications links for worldwide cellular telephone service. The satellites are placed in a plurality of inclined orbits about the earth. The orbital characteristics are tailored to maximize, by example, the coverage area of the satellites, while minimizing propagation time delays and the number of beam-to-beam and satellite-to-satellite handovers. The Horstein patent discloses additional features which are said to eliminate beam-to-beam and satellite-to-satellite handovers, thereby proposing a reduction in a likelihood of dropout.
U.S. Pat. No. 5,422,647, issued to Hirshfield et al., discloses a communications satellite payload for providing efficient communications between user devices, such as hand held cellular telephones, and terrestrial gateways. The satellite payload simultaneously supports a plurality of independent antenna beams on both the uplink and downlink for user devices employing full duplex communications. The satellite payload is carried by one satellite of a plurality of such satellites in a low earth orbit (LEO) constellation.
Another patent of interest to this invention is U.S. Pat. No. 5,448,623, issued to Wiedeman et al. This patent discloses a wireless telephone system capable of servicing a roaming wireless telephone user. The system includes, by example, a plurality of terrestrial communications links, at least one orbiting satellite, and at least one terrestrial-based gateway. The system operates by effecting communication between a terrestrial wireless telephone end user transceiver apparatus and a terrestrial communications link via a single relay satellite. The gateway effects ultimate decision on linking in cooperation with a network database to effect hand-off from a first orbiting satellite to a second orbiting satellite.